Generally, a Synchronous Optical Network (SONET) is both a standard and a set of specifications for building high speed, digital communications networks that run over fiberoptic cables while interfacing with existing electrical protocols and asynchronous transmission equipment. Such networks facilitate cutting of operating costs for regional telephone networks, while also upgrading existing networks, providing new high speed services, and interconnecting of the regional telephone network to long-distance companies and international carriers. The use of fiberoptics in such networks provides a dramatic increase in available bandwidth (currently estimated in the hundreds of gigabits per second).
One of the principal benefits of SONET is that it allows for the direct multiplexing of current network services, such as DS1, DS1C, DS2, and DS3 into the synchronous payload of Synchronous Transport Signals (STS). The STS provide an electrical interface which is used as a multiplexing mechanism within SONET Network Elements (NE). The SONET multiplexing format provides greater capacity and efficiency over traditional asynchronous arrangements. For example, in the SONET multiplexing format, the basic signal transmission rate , i.e., STS-1, operates at 51.84 million bits per second. AN STS-1 can carry 28 DS1 signals or one asynchronous DS3. STS-1 signals are then multiplexed to produce higher bit rates STS-2, STS-3, etc. SONET signal levels are also defined in terms of an optical carrier (OC). Since the bit rates are the same in each case, the bit rate of the STS-1 equals the bit rate of the OC-1 with the only difference relating to the type of signal that is being referenced. For example, if the signal is in an electrical format, it is referred to as an STS. Similarly, if the signal is in an optical format compatible with a fiber medium, the signal is referred to as an OC.
Because of the large bandwidth availability in fiber, and the growing volume of data traffic, disruptions from link and node failures due to cable cuts, for example, become increasingly serious. Network survivability has therefore become a major concern for SONET designers and has fueled interest in what is known in the art as "ring" architectures. Such architectures take advantage of the capability provided by synchronous multiplexing in SONET to eliminate the need to backhaul traffic to central hubs. Thus, at each switching office, the SONET transport node directly accesses the required time slots in the bit stream through the use of modified Add-Drop Multiplexers (ADM). The SONET ring topology permits the creation of highly survivable networks which are viewed in the communications industry as essential for obtaining business for critical data communications.
In most cases, the deployment of SONET rings results in cost savings since it is far less expensive for carriers to install a fiber ring then to deploy point-to-point links. Generally, ring topology routes working or service traffic in one direction only. If the utilized fiber fails, traffic is rerouted on a protection fiber and travels in the opposite direction. In this manner, working traffic bypasses the failure to reach the intended destination. In addition, bidirectional ADMs are also used to allow signals to be rerouted back through an ADM to reach a destination terminal.
One example of an arrangement for assigning inter-nodal traffic loads to channels in SONET rings is provided by commonly-owned U.S. Pat. No. 5,564,021 to Qiu et al. In this SONET planning arrangement, a mixed integer program (MIP) is used to model the cost of terminal multiplexers (TM), ADMs, and corresponding interface ports necessary to route the desired traffic loads. While such an arrangement improves traffic routing, a need exists for a SONET optimizing tool which provides even more accurate modeling and cost effective network demand assessment and management.